A cutoff in the X-ray fluctuation power density spectrum of the Seyfert 1 galaxy NGC 3516

During 1997 March-July, the Rossi X-Ray Timing Explorer observed the bright , strongly variable Seyfert 1 galaxy NGC 3516 once every similar to 12.8 ho urs for 4.5 months and nearly continuously (with interruptions due to South Atlantic Anomaly passage but not Earth occultation) for a 4.2 day period i n the middle. These were followed by ongoing monitoring once every similar to 4.3 days. These data are used to construct the first well-determined X-r ay fluctuation power density spectrum (PDS) of an active galaxy to span mor e than four decades of usable temporal frequency. The PDS shows no signs of any strict or quasi-periodicity but does show a progressive flattening of the power-law slope from -1.74 at short timescales to -0.73 at longer times cales. This is the dearest observation to date of the long predicted cutoff in the PDS. The characteristic variability timescale corresponding to this cutoff temporal frequency is similar to 1 month. Although it is unclear ho w this timescale may be interpreted in terms of a physical size or process, there are several promising candidate models. The PDS appears similar to t hose seen for Galactic black hole candidates such as Cyg X-1, suggesting th at these two classes of objects with very different luminosities and putati ve black hole masses (differing by more than a factor of 10(5)) may have si milar X-ray generation processes and structures.